The present invention relates generally to methods for suppressing fuming in molten steel and more particularly to methods for suppressing fuming in molten steel containing fume-generating ingredients.
The fume-generating, alloying ingredients to which the present invention is directed comprise bismuth, lead, aluminum and manganese. These fume-generating ingredients are added to a bath of molten steel typically contained within a ladle, or in the case of strand cast steel, sometimes in the tundish of a continuous casting apparatus.
A bath of molten steel without any of these fume-generating ingredients emits fumes in the form of iron vapor which is oxidized by the oxygen in the atmosphere directly above the bath causing formation of iron oxides. The conversion of iron vapors to iron oxides depletes the iron vapors above the bath and causes further generation of iron fumes to replenish the iron fumes converted to iron oxides. In such a case, fuming is maintained by the presence of oxygen in the atmosphere directly above the bath and such fuming is called forced vaporization.
The prior art solution for dealing with iron fuming has been to remove or eliminate the oxygen immediately above the bath of molten steel. This has been accomplished by combusting the oxygen with natural gas introduced through a conventional lance to the locale directly above the molten steel bath where the oxygen is located. By removing the oxygen from above the molten steel bath in this manner, the generation of iron oxide fume has been drastically reduced.
In another technique, called passive control, the oxygen above the molten steel bath is consumed by reaction with iron vapor and the resulting oxygen-depleted atmosphere is contained or confined above the bath.
Prior art studies have indicated that, for pure metals, the fume generation rate is directly related to the partial pressure of oxygen and is independent of temperature and a particular metal's vapor pressure.
Fume-generating, alloying ingredients such as bismuth, lead, manganese and aluminum have vapor pressures substantially greater than that of iron. Based on the teachings of the prior art, one would conclude that the fume generation rate for these elements, when added as alloying ingredients to molten steel, should be the same as that for molten iron for a given partial pressure of oxygen. Accordingly, if the partial pressure of oxygen is reduced to a level at which fuming of iron from the molten steel is unobjectionable, fuming due to these alloying ingredients should also be unobjectionable. Such, however, has proved to be not the case.